1. Field of the Invention
An aspect of the present invention generally relates to a mobile communication system. More particularly, an aspect of the present invention relates to a method and apparatus for transmitting/receiving channel quality information in a Multiple Input Multiple Output (MIMO)-Orthogonal Frequency Division Multiplexing (OFDM) mobile communication system.
2. Description of the Related Art
MIMO is essential to enable high-speed mobile data services in a broadband mobile communication system. A MIMO system is an antenna system with multiple inputs and multiple outputs. Since the MIMO system can transmit different information from each antenna, the MIMO system can increase the amount and reliability of processed information. In contrast, 2nd Generation (2G) and 3rd Generation (3G) mobile communication systems have limitations in fast processing of a wide range of data due to the use of a single transmit/receive antenna.
Along with MIMO, OFDM has emerged as a promising post-3G mobile communication technology. In OFDM, time and frequency resources are divided for allocation to users. OFDM is favorable to broadband transmission and allocates time and frequency resources according to users' demands. Due to these benefits, OFDM was adopted for Digital Audio Broadcasting (DAB) and Digital Video Broadcasting (DVB) in Europe, and also approved as a standard for Wireless Local Area Network (WLAN). Compared to OFDM, Code Division Multiple Access (CDMA) supports no more than up to 144 kbps in a mobile environment and is not effective in processing a large amount of wireless data.
The current wireless communication systems attempt to achieve high-quality transmission of data and large multimedia data transmission with limited frequency resources. Therefore, techniques for transmitting a large amount of data with limited frequency resources have been proposed. A major technique is the MIMO system that can transmit more data without using more frequency resources.
However, the MIMO system is vulnerable to Inter-Symbol Interference (ISI) and frequency selective fading involved in high-speed data transmission. To overcome this shortcoming, MIMO is used in combination with OFDM. An OFDM system divides high-rate data streams into low-rate data streams by parallel data processing and transmits the low-rate data streams on a plurality of subcarriers simultaneously. The use of low-rate parallel subcarriers increases a symbol period, thereby decreasing ISI. Since guard intervals are used, the ISI is almost perfectly cancelled.
As the OFDM system uses multiple subcarriers, it is robust against frequency selective fading. Therefore, the combination of MIMO and OFDM compensates for the shortcomings of MIMO, while realizing the advantages of MIMO. The MIMO system typically has a plurality of transmitter antennas and a plurality of receiver antennas. A system using MIMO and OFDM in combination is called a MIMO-OFDM system.
In the MIMO system, a transmitter selects at least one appropriate transmitter antenna for a Mobile Station (MS) with one or more receiver antennas. To do so, the transmitter, for example, a Base Station (BS) needs feedback of channel quality information from the MS. In the nature of using a plurality of transmit/receive antennas, the total amount of channel quality information is very large, thus imposing a large overhead on uplink signaling. Especially in the MIMO-OFDM system, the feedback information further includes channel quality information about a plurality of individual subcarriers (i.e. subbands), adding to the uplink signaling overhead. Accordingly, many techniques are under development to reduce the amount of feedback information, while enabling the transmitter to efficiently map data streams to the transmitter antennas.
The simplest method for transmitting feedback information in the MIMO-OFDM system is to transmit channel quality information about all subbands or N best subbands, which results in a very large signaling overhead. Another method is to select a few subbands and schedule the selected subbands only. However, this method gives up an advantage of OFDM (i.e. frequency selection diversity). A third method is to generate channel quality information on a subband group basis, each subband grouping having M adjacent subbands. Considering a grouping size dominantly affects signaling efficiency, an optimal grouping size should be adapted according to channel status. Moreover, the third method suffers resource use inefficiency when the channel statuses of adjacent subbands are different.